In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA) system, Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few.
LTE is a Frequency Division Multiplexing technology wherein Orthogonal Frequency Division Multiplexing (OFDM) is used in downlink (DL) transmissions from a radio base station to a user equipment (UE). Single Carrier-Frequency Domain Multiple Access (SC-FDMA) is used in uplink (UL) transmissions from the user equipment to the radio base station. Services in LTE are supported in the packet switched domain.
In a time domain, according to LTE, one subframe of 1 ms duration is divided into 12 or 14 OFDM, or SC-FDMA, symbols, depending on a configuration of the subframe. One OFDM or SC-FDMA symbol comprises a number of sub carriers in the frequency domain, depending on a channel bandwidth and configuration. One OFDM or SC-FDMA symbol on one sub carrier is referred to as a Resource Element (RE).
In LTE no dedicated data channels are used, instead shared channel resources are used in both downlink and uplink. These shared resources, Downlink Shared Channel (DL-SCH) and Uplink Shared Channel (UL-SCH), are each controlled by one scheduler in the radio base station, which scheduler assigns different parts of the downlink and uplink shared channels to different user equipments for reception and transmission, respectively.
The schedulers are in full control of in which subframe a user equipment should receive a DL-SCH transmission and which subframe the user equipment is allowed to transmit on the UL-SCH. Scheduling decisions are sent to the user equipment as downlink assignments and uplink grants. Downlink assignment information and uplink grants are transmitted in Downlink Control Information (DCIs) using Layer 1/Layer 2 control signalling. A downlink assignment message indicates if there is data to be received for the user equipment on the DL-SCH.
Dynamic scheduling enables multiple user equipments to share all, or parts of, available frequency resources in one subframe; all, or parts of, frequency resources are assigned to one user equipment; and no user equipments are allocated any frequency resources.
A resulting resource allocation over time and frequency depends both on properties of the user equipments in the system, i.e. the number of user equipments, traffic models of the user equipments, radio channel characteristics, and on an algorithm implementing a scheduling functionality. A strategy defining in which way resources in time and frequency are allocated to a set of user equipments is commonly referred to as a scheduling algorithm.
Scheduling algorithms that prioritize users which have a good channel or radio condition perform so called channel dependent scheduling. Channel dependent scheduling utilizes the multi user diversity, where multiple users are spread out in the cell and thus experience different channel quality dips at different frequencies and at different times. For example, a pure channel dependent scheduling algorithm typically prioritizes the mobile terminal which has a good channel or radio condition. As a result, the cell throughput will be maximized, however with the cost of starving mobile terminals having bad channel conditions. Channel dependent scheduling is therefore said to be unfair.
Proportional fair (PF) scheduling adds control of an overall fairness in the radio communications network by prioritizing user equipments not only based on a channel quality of the user equipment but also on an average rate of a transmission. The overall fairness of the scheduling is controlled by steering a proportion of the two factors, i.e. instantaneous channel quality and an average rate of transmission. The PF scheduling strategy is able to utilize channel variations to improve overall cell throughput while still ensuring reasonable fairness between UEs.
As previously described, LTE enables dynamic scheduling where resources are orthogonal in the frequency domain, thus enabling channel dependent scheduling to be used in both time and frequency. Prioritizing which resources in the frequency domain that should be allocated to a UE is called Frequency Selective Scheduling (FSS). If applied in an LTE scenario, an optimal frequency selective scheduler would only assign resources to a UE where its channel quality is high. The channel quality may be estimated in the radio base station based on UE reported CQI and/or reception of uplink reference signals.
One way of implementing FSS is to employ proportional fair scheduling, where the channel quality measure is based on variations in both time and frequency, that is, proportional fair in time and frequency. PF in time and frequency is believed to ensure higher cell throughput and fairness among UEs.
Frequency selective scheduling algorithms of today require immense computation power to find an optimized solution. One example of such solutions is to compute/determine the number of bits as a measure of the channel quality. That requires performing link adaptation before making a scheduling decision which is associated with very high computational complexity. A simple channel quality metric is crucial to realize frequency selective scheduling in practice. Examples of channel quality measures, which are more feasible for implementation, are Gain to Interference and Noise Ratio (GINR) or Signal to Interference and Noise Ratio (SINR). However, methods using both these typical channel quality measures have associated problems. Using GINR, the scheduler in the radio base station tends to favour UEs in a centre of the cell. SINR is a good channel quality measure, but it is difficult to estimate SINR before knowing where and how many resource blocks are allocated.